Inorganica Chimica Acta 361 (2008) 3659–3662

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Inorganica Chimica Acta

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Magnetic and photo-magnetic properties of Co dinuclear complexes

Osamu Sato a,*, Satoshi Miura a, Hiroyuki Maruyama a, Yanjuan Zhang a, Dayu Wu a, Wen Zhang a,Haitao Xu a, Ryotaro Matsuda a, Haoling Sun b, Jun Tao c

a Institute for Materials Chemistry and Engineering, Kyushu University, 6-1 Kasuga, 816-8580 Fukuoka, Japanb Department of Chemistry, Beijing Normal University, Beijing 100875, PR Chinac Department of Chemistry, Xiamen University, Xiamen 361005, PR China

a r t i c l e i n f o a b s t r a c t

Article history:Received 23 February 2008Received in revised form 18 March 2008Accepted 18 March 2008Available online 23 March 2008

Dedicated to Dante Gatteschi.

Keywords:Valence tautomerismPhoto-induced electron transferLong lifetime

0020-1693/$ - see front matter � 2008 Elsevier B.V. Adoi:10.1016/j.ica.2008.03.089

* Corresponding author.E-mail address: sato@cm.kyushu-u.ac.jp (O. Sato).

The magnetic and photo-magnetic properties of Co dinuclear compounds were studied. The Cocompounds, [{Co(tpa)}2(dhbq)](BF4)3 � 2H2O and [{Co(tpa)}2(dhbq)](ClO4)3 � 2H2O, have the structure[(tpa)CoIII-LS–(l-dhbq)3�–CoIII-LS(tpa)]3� (where LS, H2dhbq, tpa denote low-spin, 2,5-di-hydroxy-1,4-benzoquinone and tris(2-pyridylmethyl)amine, respectively) at room temperature. On heating, thesecompounds exhibit valence-tautomeric inter-conversion; [(tpa)CoIII-LS–(l-dhbq)3�–CoIII-LS(tpa)]3�

¡

[(tpa)CoII-HS–(l-dhbq)2�–CoIII-LS(tpa)]3� (where HS denotes high-spin). Furthermore, both of these com-pounds exhibit photo-induced valence tautomerism at low temperature (<60 K). These results show thatthe electronic structures of these complexes can be controlled by modifying the counter anions.

� 2008 Elsevier B.V. All rights reserved.

1. Introduction

There has been great interest in the study of the magnetic prop-erties of molecular compounds [1]. One of the current popular top-ics in this field is the development of molecular nano-magnets. Inparticular, Gateschi and co-workers have reported the develop-ment of single molecule magnets and single chain magnets, whichhave attracted great attention recently [2,3]. Since their discovery,a large number of molecular nano-magnets have been studied [4].

Another important field is the preparation of magnetic com-pounds that respond to light [5–8]. Examples of this phenomenonare spin crossover complexes that exhibit light-induced excitedspin state trapping (LIESST) effects [9]. Iron(II) and Iron(III) LIESSTcomplexes have already been reported [9,10]. Another group ofinteresting compounds are molecular photo-magnets [11]. Prus-sian blue analogues, octa-cyanide-based magnets, Mn-TCNE, etc.have been reported to produce photo-responsive magnets [11–13]. More recently, photo-induced valence tautomerism has beenobserved in Co valence-tautomeric compounds [14–16]. So far,mono-nuclear, di-nuclear and poly-nuclear Co complexes thatshow photo-induced valence tautomerism have been reported[14,17–20].

Following these principles, we have been carrying out studies ofmolecular magnetic compounds, and we have recently succeeded

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in developing a novel single molecule magnet and a single chainmagnet [21,22]. Furthermore, we have reported an Fe(III) LIESSTcomplex [10,23], an Fe(II) LIESST complex with a high relaxationtemperature (130 K) [24], and a molecular photo-magnet, basedon an Fe–Co Prussian blue compound [11,25–28]. More recently,we reported a Co complex that shows photo-induced valence tau-tomerism [14,16,17,19,29–32]. That is, when the Co valence-tauto-meric complex was excited, a metastable state with an extremelylong life-time was generated. As a result, an appreciable increasein magnetization was observed.

However, the number of such photo-responsive valence-tauto-meric compounds that have been identified is very small, and rel-atively few examples have been reported. Hence, we decided tostudy the magnetic properties of several Co dinuclear complexes,and we found that [{Co(tpa)}2(dhbq)](BF4)3 � 2H2O and [{Co(t-pa)}2(dhbq)](ClO4)3 � 2H2O exhibit thermally- and photo-inducedvalence tautomerism.

2. Experimental

[{Co(tpa)}2(dhbq)](BF4)3 � 2H2O (1) (Fig. 1): A solution ofCoCl2 � 6H2O (2.0 mmol) and tpa (2.0 mmol) in MeOH (60 mL)was added to a solution of H2dhbq (1.0 mmol) and Et3N (0.28 mL,2.0 mmol) in MeOH (10 mL). The resulting suspension was stirredat room temperature for 5 min, after which the solution was re-fluxed for 30 min and an aqueous solution (10 mL) of KBF4

(4.0 mmol) was added. Small, block-like red crystals were obtained

III IIIO

O

N

N

NN Co

O

O N

N

NNCo

T, hν

Compound 1

Compound 2

(ClO4)3(ClO4)3

III IIIO

O

N

N

NN Co

O

O N

N

NNCo

T, hν

(BF4)3(BF4)3

O

O

N

N

NN Co

O

O N

N

NNCoIII II

O

O

N

N

NN Co

O

O N

N

NNCoIII II

.

.

Fig. 1. Chemical structure of compounds 1 and 2. They exhibit thermally- and photo-induced valence tautomerism.

3660 O. Sato et al. / Inorganica Chimica Acta 361 (2008) 3659–3662

by slow evaporation of the resulting solution, which was washedseveral times with water and methanol. Yield: �16%. Anal. Calc.for C42H42B3Co2F12N8O6: C, 44.52; H, 3.74; N, 9.89. Found: C,44.32; H, 3.76; N, 9.85%.

[{Co(tpa)}2(dhbq)](ClO4)3 � 2H2O (2) (Fig. 1): a solution ofCoCl2�6H2O (2.0 mmol) and tpa (2.0 mmol) in MeOH (60 mL) wasadded to a solution of H2dhbq (1.0 mmol) and Et3N (0.28 mL,2.0 mmol) in MeOH (10 mL). The resulting suspension was stirredat room temperature for 5 min. The solution was then refluxed for30 min and an aqueous solution (10 mL) of NaClO4 (4.0 mmol) wasadded. Small, needle-like red crystals were obtained by slow evap-oration of the resulting solution, which was washed several timeswith water and methanol. Yield: �4.0%. Anal. Calc. forC42H42Cl3Co2N8O18: C, 43.08; H, 3.62; N, 9.57. Found: C, 43.14; H,3.67; N, 9.49%.

The magnetic susceptibilities of these complexes were mea-sured using a superconducting quantum interference device(SQUID) magnetometer (Quantum Design MPMS-5S). Their infra-red (IR) spectra were measured by the KBr method using a JASCOFT/IR-600. UV–Vis absorption spectra were measured using a UV-3100 (Shimazu) spectrophotometer. A laser-diode pumpedNd:YAG laser (Crystal Laser GCL-150-M) with a wavelength of532 nm was used as the light source.

0.2

0.4

0.6

0.8

1

1.2

1.4

0 50 100 150 200 250 300 350 400

1st heating

1st cooling

2nd heating

T /K

χ mT

/ cm

3K

mol

-1

Fig. 2. vmT vs. T curve of 1.

3. Results and discussion

The magnetic properties of 1 were measured in an external fieldof 0.5 T. The vmT versus T curve is shown in Fig. 2, where vm is themagnetic susceptibility and T is the temperature. The vmT value at5 K is equal to 0.32 cm3 mol�1 K, meaning that the redox state of 1can be expressed as [(tpa)CoIII-LS(l-dhbq)3�CoIII-LS(tpa)](BF4)3 �2H2O. On warming, the value of vmT increases above 350 K. ThevmT value at 400 K was 1.24 cm3 mol�1 K. This means that va-lence-tautomeric conversion was induced in the complex. The va-lence-tautomeric behavior can be expressed as follows:

½ðtpaÞCoIII-LSðl-dhbqÞ3�CoIII-LSðtpaÞ�3�

¡½ðtpaÞCoII-HSðl-dhbqÞ2�CoIII-LSðtpaÞ�3�

This behaviour is consistent with the (PF6) salt of the Co complex[19]. Gravimetric analysis showed that the water molecules are re-moved at around 370 K in the initial warming process. Hence, thetemperatures at which valence-tautomeric transitions occur are dif-ferent between the first cycle and the second cycle. The phase tran-sition temperature of 1 during the second cycle is ca. 370 K. On theother hand, the phase transition temperature of [(tpa)CoIII-LS(l-dhbq)3�CoIII-LS(tpa)](PF6)3 was ca. 300 K [19]. A shift in the phasetransition temperature of 70 K was induced by replacing (PF6) with(BF4).

The magnetic properties of 2 are shown in Fig. 3. The vmT valueat 300 K is ca. 0.4 cm3 mol�1 K, meaning that the redox state of 2 is

Fig. 3. vmT vs. T curve of 2.

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 20 40 60 80 100

Before irradiation

After irradiation

hν

T / K

χ mT

/ cm

3K

mol

-1

Fig. 5. Magnetic properties of 1 before and after photo-irradiation.

O. Sato et al. / Inorganica Chimica Acta 361 (2008) 3659–3662 3661

[(tpa)CoIII-LS(l-dhbq)3�CoIII-LS(tpa)] (ClO4)3 � 2H2O. The vmT valueabruptly increases at 390 K. The vmT value at 400 K is ca.2.5 cm3 mol�1 K, which is consistent with the expected value forthe [(tpa)CoIII-LS(l-dhbq)2�CoII-HS(tpa)]3� state. This means thatthe valence-tautomeric inter-conversion, [(tpa)CoIII-LS(l-dhbq)3�CoIII-LS(tpa)]3�

¡ (tpa)CoII-HS(l-dhbq)2�CoIII-LS(tpa)]3�, wasinduced in this complex too. On cooling, the magnetization valueabruptly decreased at around 360 K, reaching a value of ca.0.5 cm3 mol�1 K. When the temperature was increased again upto 400 K, the abrupt increase was observed at ca. 380 K. Whenthe temperature was decreased, the abrupt decrease occurred at360 K. Therefore, an important characteristic to note is that hyster-esis was observed in complex 2: The hysteresis width is ca 20 K.The reason why the first cycle is different from the second andthird cycles is that water molecules are removed from the crystalduring the first cycle, as confirmed by gravimetric analysis. Wehave reported that [(tpa)CoIII-LS(l-dhbq)3�CoIII-LS(tpa)](PF6)3 showshysteresis at around room temperature [19]. However, this is atwo-step transition. Only 50% of the moieties show transitions ataround room temperature. On the other hand, almost 100% ofthe moieties undergo valence-tautomeric transition in 2 with(ClO4), which is distinguishable from the (PF6) salt.

It has been reported that the Co valence-tautomeric compoundsshow photo-induced valence tautomerism [15,16,18,19]. Hence,we have studied the photo-magnetic behavior of compounds 1and 2. The absorption spectrum for 2 is shown in Fig. 4. A strongabsorption peak was observed at around 550 nm, which is ascrib-able to the ligand-to-metal charge transfer (LMCT) band of thesecomplexes. In order to excite the LMCT band, a diode-pumpedNd:YAG laser with a wavelength of 532 nm was used as a lightsource, and we measured the photo-magnetic properties. Lightwas guided into the SQUID magnetometer via an optical fiber in or-der to study the photo-effects, and the powder sample was sup-ported on a commercial transparent adhesive tape.

When complex 1 was irradiated at 5 K, an increase in the mag-netization value was clearly observed. Fig. 5 shows the photo-mag-netic properties of 1. As shown in the figure, the magnetizationvalue increased after irradiation. This shows that, as in the caseof other Co valence-tautomeric compounds [16], complex 1 exhib-ited photo-induced valence tautomerism. The photo-induced pro-cess can be expressed as follows:

½ðtpaÞCoIII-LSðl-dhbqÞ3�CoIII-LSðtpaÞ�3�

¡½ðtpaÞCoII-HSðl-dhbqÞ2�CoIII-LSðtpaÞ�3�

It is believed that the spin-allowed [(tpa)CoII-LS(l-dhbq)2�CoIII-

LS(tpa)]3� state is induced by exciting the LMCT band. After the exci-tation, it relaxes to the [(tpa)CoII-HS(l-dhbq)2�CoIII-LS(tpa)]3� statedue to spin–orbit coupling. The resulting metastable state, [(tpa)-

300 400 500 600 700 800

Abs

orba

nce

λ / nm

Fig. 4. Absorption spectra for 2.

CoII-HS(l-dhbq)2�CoIII-LS(tpa)]3�, was trapped at 5 K. Note that thespin-allowed transition described above is symmetry forbidden.Hence, the photo-process should be further studied.

When the temperature was increased after photo-irradiation,the magnetization value decreased and recovered to the originallevel at around 60 K. This means that back electron-transfer fromCoII-HS to (dhbq)2� was induced via a thermal activation process.That is, the metastable state recovered to the original ground state,[(tpa)CoIII-LS(l-dhbq)3�CoIII-LS(tpa)]3�.

Similar photo-magnetic effects were observed for 2. Fig. 6shows the photo-magnetic properties of 2. The photo-induced va-lence-tautomeric behavior is [(tpa)CoIII-LS(l-dhbq)3�CoIII-

LS(tpa)]3�¡ [(tpa)CoII-HS(l-dhbq)2�CoIII-LS(tpa)]3�. When the tem-

perature was increased after photo-irradiation, the metastablestate completely recovered to the [(tpa)CoIII-LS(l-dhbq)3�CoIII-

LS(tpa)]3� state at around 60 K, meaning that reverse valence tau-tomerism was induced.

These measurements show that both 1 and 2 are photo-respon-sive compounds. That is, they exhibit photo-induced magnetiza-tion effects due to valence-tautomeric inter-conversion. Theresulting meta-stable state can be reversed to the ground stateby heating the sample. A general feature of photo-induced valencetautomerism in Co complexes is that the magnetization value afterphoto-irradiation is much lower than the value expected for 100%conversion [16]. It has been reported that only the surface layer re-sponds to light irradiation [18]. As shown in Figs. 5 and 6, the mag-netization value after irradiation does not come close to reaching

0.6

0.8

1

1.2

1.4

0 20 40 60 80 100

Before irradiation

After irradiation

hν

T / K

χ mT

/ cm

3K

mol

-1

Fig. 6. Magnetic properties of 2 before and after photo-irradiation.

3662 O. Sato et al. / Inorganica Chimica Acta 361 (2008) 3659–3662

the expected value for 100% conversion. This means that photo-in-duced valence tautomerism occurs only at the surface layer forcompounds 1 and 2.

In summary, we have reported the magnetic properties of Codinuclear complexes. The Co complexes 1 and 2 exhibited valencetautomerism. In contrast to the case of [(tpa)CoIII-LS(l-dhbq)2�CoII-

HS(tpa)](PF6)3, both 1 and 2 showed a one-step transition. Com-pound 1 does not undergo hysteresis, while compound 2 exhibiteda hysteresis loop with a width of 20 K. Furthermore, we found thatboth 1 and 2 undergo photo-induced valence tautomerism whenthe LMCT band is excited by light. This effect can be expressedas [(tpa)CoIII-LS(l-dhbq)3�CoIII-LS(tpa)]3�

¡ [(tpa)CoII-HS(l-dhbq)2�-CoIII-LS(tpa)]3�. This means that 1 and 2 are photo-responsivecomplexes.

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